Determining a rock formation content

ABSTRACT

Techniques for determining an amount of chert in a rock formation include identifying first logging data for a selected zone of a first subterranean formation that includes chert, quartz, and zircon; identifying second logging data for a second subterranean formation that is different than the first subterranean formation, the second subterranean formation including chert, quartz, and zircon; determining a first volumetric ratio of quartz to zircon in the selected zone based on the first logging data; determining a second volumetric ratio of quartz to zircon based on the second logging data; determining a maximum value of the second volumetric ratio; and calculating, based on the first and second volumetric ratios and the maximum value of the second volumetric ratio, a volumetric percentage of chert in the selected zone.

TECHNICAL FIELD

This disclosure relates to determining a content of a rock formationand, more particularly, determining an amount of chert in a rockformation that includes quartz.

BACKGROUND

Chert is a rock that has the same or similar elemental composition toquartz. Thus, conventional logging tools that measure density, neutron,and resistivity may only identify chert as quartz across a subterraneanformation rather than chert, itself. Chert, however, relative to manyother rocks in hydrocarbon bearing formations, has a high rock strength,and the presence of chert in subterranean formations may adddifficulties to drilling and completion operations.

SUMMARY

This disclosure describes implementations of methods and systems fordetermining an amount of chert in a subterranean formation. In someaspects, the amount of chert is determined according to conventionallogs that identify volumetric ratios of quartz (with a similar elementalstructure as chert) and zircon within the selected formation, as well asbaseline logs from subterranean formations independent of the selectedformation and particular zones within the formation (e.g., knownhydrocarbon bearing zones).

In an example implementation, techniques for determining an amount ofchert in a rock formation include identifying first logging data for aselected zone of a first subterranean formation that includes chert,quartz, and zircon; identifying second logging data for a secondsubterranean formation that is different than the first subterraneanformation, the second subterranean formation including chert, quartz,and zircon; determining a first volumetric ratio of quartz to zircon inthe selected zone based on the first logging data; determining a secondvolumetric ratio of quartz to zircon based on the second logging data;determining a maximum value of the second volumetric ratio; andcalculating, based on the first and second volumetric ratios and themaximum value of the second volumetric ratio, a volumetric percentage ofchert in the selected zone.

An aspect combinable with the example implementations includescalculating an absolute volume of the chert in the selected zone basedon the volumetric percentage of the chert in the selected zone.

In another aspect combinable with any of the previous aspects,calculating the absolute volume of the chert in the selected zoneincludes determining a total volume of the quartz in the selected zone;and multiplying the volumetric percentage of the chert in the selectedzone by the total volume of the quartz in the selected zone.

In another aspect combinable with any of the previous aspects, the firstsubterranean formation includes a marine deposition subterraneanenvironment, and the second subterranean formation includes a non-marinedeposition subterranean environment.

In another aspect combinable with any of the previous aspects,calculating, based on the first and second volumetric ratios and themaximum value of the second volumetric ratio, a volumetric percentage ofthe chert in the selected zone includes solving the equation

${P_{chert} = \frac{R_{zone} - R_{Baseline}}{R_{\max}}},$where P_(chert) is the volumetric percentage of the chert in theselected zone, R_(zone) is the first volumetric ratio, R_(Baseline) isthe second volumetric ratio, and R_(max) is the maximum value of thesecond volumetric ratio.

In another aspect combinable with any of the previous aspects, theselected zone includes a Qusaiba geological formation.

Another aspect combinable with any of the previous aspects furtherincludes displaying, on a graphical user interface, the calculatedvolumetric percentage of chert in the selected zone.

In another aspect combinable with any of the previous aspects,displaying the calculated volumetric percentage of chert in the selectedzone includes displaying the calculated volumetric percentage of chertas a function of depth between a shallowest depth of the selected zoneand a deepest depth of the selected zone.

Another aspect combinable with any of the previous aspects furtherincludes recommending an adjustment to a drilling or completionoperation based at least in part on the calculated volumetric percentageof chert in the selected zone.

Another aspect combinable with any of the previous aspects furtherincludes receiving the first logging data from a logging tool in awellbore formed through the selected zone of the first subterraneanformation.

In another aspect combinable with any of the previous aspects, thelogging tool includes a logging-while-drilling (LWD) tool.

The example implementation and aspects thereof may be implemented insystems, computer-implemented methods, and non-transitory computerreadable media. For example, a system of one or more computers can beconfigured to perform particular actions by virtue of having software,firmware, hardware, or a combination of them installed on the systemthat in operation causes or cause the system to perform the actions. Oneor more computer programs can be configured to perform particularactions by virtue of including instructions that, when executed by dataprocessing apparatus, cause the apparatus to perform the actions.

Implementations of methods and systems according to the presentdisclosure may include one or more of the following features. Forexample, methods and systems for determining an amount of chert in asubterranean formation may utilize conventional logging techniques todetermine the amount of chert. Thus, the disclosed methods and systemsmay eliminate or help eliminate a need to have additional core samplesof the subterranean formation studied at a laboratory to determine theamount of chert in the formation. Further, the disclosed methods andsystems may determine an amount of more cost and time efficient thussaving time and money.

The details of one or more implementations of the subject matterdescribed in this disclosure are set forth in the accompanying drawingsand the description below. Implementations may be in the form ofsystems, methods, apparatus, and computer-readable media. For example, asystem of one or more computers can be configured to perform particularactions by virtue of having software, firmware, hardware, or acombination of them installed on the system that in operation causes orcause the system to perform the actions. One or more computer programscan be configured to perform particular actions by virtue of includinginstructions that, when executed by data processing apparatus, cause theapparatus to perform the actions. Features, aspects, and advantages ofthe subject matter will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an example system for determiningan amount of chert in a geological formation according to the presentdisclosure.

FIG. 2 is a flowchart that illustrates an example method for determiningan amount of chert in a geological formation according to the presentdisclosure.

FIG. 3 is a graph that illustrates logging data and calculated valuesused in the example method of FIG. 2.

FIG. 4 is a graph that illustrates rock strength measurements using ascratch test performed on a core sample of a rock formation thatcontains chert used in a laboratory test to confirm the example methodof FIG. 2.

FIG. 5 is a photomicrograph of a source rock formation that containschert that illustrates a laboratory test to confirm the example methodof FIG. 2.

FIG. 6 illustrates focus ion beam and scanning electron microscopyimages from samples of a rock formation that contains chert used in alaboratory test to confirm the example method of FIG. 2.

FIG. 7 illustrates a schematic diagram of a computing system for acomputer-implemented method for an image-based analysis of thegeological thin section.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of an example system 100 fordetermining an amount of chert in a geological formation. Generally,FIG. 1 illustrates a portion of one embodiment of the system 100 inwhich a logging tool, such as a logging tool 118, may generate or recordlogging data that is used to determine an amount of chert in aparticular portion of a subterranean zone. In some cases, as shown inFIG. 1, the logging tool 118 is part of or coupled to a bottom holeassembly (BHA) 120 that includes a drilling bit or other wellboreformation tool (e.g., laser or otherwise). The generated or recordedlogging data is used, in this example, by a control system 122 tocalculate the amount of chert based at least in part on data thatdescribes an amount of quartz and zircon within the particulargeological formation.

In some aspects, system 100 may be used to quantify the amount of chertpresent in the particular geological formation based, at least in part,on geochemical volume ratio data derived from the logging data taken,e.g., within the system 100, in another wellbore system, or from anothersubterranean formation separate from a subterranean formation shown inFIG. 1. For instance, because chert is comprised of the same or verysimilar elemental composition as quartz, logging data, such as density,neutron, and gamma data may solely identify a presence of quartz withinthe formation, rather than chert and quartz as separate components. Byutilizing a system such as system 100, the need to have core samples ofthe particular formation taken to determine the presence of chert may bereduced or eliminated.

As shown, the system 100 accesses a subterranean formation 110, andprovides access to hydrocarbons located in such subterranean formation110. In an example implementation of system 100, the system 100 may beused for a drilling operation in which the BHA 120 that includes adrilling bit may be used to form a wellbore 114 (e.g., with drill string116, wireline, or otherwise) that extends through the subterraneanformation 110 to a particular zone 112 of the formation 116. In anotherexample implementation of system 100, the system 100 may be used for acompletion, for example, hydraulic fracturing, operation, prior to whichthe logging tool 118 may be coupled to the drilling string 116 (orwireline) without the BHA 120.

As illustrated in FIG. 1, an implementation of the system 100 includes adrilling assembly 102 deployed on a terranean surface 12. The drillingassembly 102 may be used to form the wellbore 20 extending from a subseasurface 108 and through one or more geological formations in the Earth.One or more subterranean formations, such as subterranean zone 110, arelocated under the subsea surface 108.

In this example implementation, the drilling assembly 102 is deployed ona body of water 106 (e.g., ocean, gulf, sea) rather than a terraneansurface. The drilling assembly 102, in this figures, is shown as asemi-submersible that floats on the body of water 106 while beinganchored to the subsea surface 108 with one or more tethers 104.However, the present disclosure contemplates that the drilling assembly102 can also be a drillship, drilling rig, or other drilling assemblyeither on a body of water or a terranean surface. In short, the presentdisclosure contemplates that the system 100 may be implemented on landand water surfaces and contemplates forming, developing, and completingone or more wellbores 114 from either or both locations.

In the example implementation of the system 100, the wellbore 114 is anopen hole completion (e.g., without casing). The illustrated loggingtool 118 (with or without the BHA 120) may traverse the wellbore 114(e.g., from the subsea surface 108 to a bottom of the wellbore 115within or past the zone 112 of the subterranean formation 110.Generally, the logging tool 118 (which may be a logging-while-drilling(LWD) tool) measure properties of the geological formation of thesubterranean zone 110 while traversing the wellbore 114. Propertiesinclude, for example, resistivity, porosity, sonic velocity, gamma ray,and other properties which can define the characteristics of theformation, such as type of rock. For instance, the logging tool 118 candetect rock morphology (e.g., type of rock) according to certainproperties, such as, for example, density, to distinguish between quartzand zircon in the subterranean formation 110 and, more particularly, theselected zone 112 of the formation 110. As noted, however, rockproperties may not distinguish chert from quartz given these similarityof the elemental composition of these two rocks.

As shown, the drilling assembly 102 (or other portion of the system 100)includes a control system 122, for example, microprocessor-based,electro-mechanical, or otherwise, that may receive measured logging datafrom the logging tool 118 (or may identify previously recorded andstored logging data associated with the subterranean formation 110 andselected zone 112. In some aspects, the control system 122 may receive acontinual or semi-continual stream of logging data from the logging tool118 and, in some aspects, adjust the BHA 120 based on a determinedamount of chert in the zone 112 of the subterranean formation 110. Insome aspects, the control system 122 may receive a continual orsemi-continual stream of logging data from the logging tool 118 and, insome aspects, recommend actions to take for drilling or completionoperations in the wellbore 114 based on the determined amount of chertin the zone 112 of the subterranean formation 110.

The control system 122 may store (e.g., at least transiently) thelogging data from the logging tool 118 in a computer-readable media thatis communicably coupled to or a part of the system 122. The controlsystem 122 may also store normalized logging data that has been taken(or previously taken) from a subterranean formation that is differentfrom the subterranean formation 110. For example, the normalized loggingdata may be logging data from nearby (e.g., the wellbore 114 orformation 110) non-related formations. In the illustrated example, forinstance, the normalized logging data may be from a subterraneanformation that has a different source rock from the subterraneanformation 110. Further, in the illustrated example, for instance, thenormalized logging data may be from a subterranean formation that is nota marine depositional environment but rather, is located under aterranean surface as opposed to a subsea surface. In some aspects,subterranean formation 110 may be a Qusaiba shale formation which iscomprised, among other rocks, of quartz, chert, and zircon minerals.Thus, normalized logging data may be logging data from a formation thatis different or distinct from a Qusaiba shale formation.

FIG. 2 is a flowchart that illustrates an example method 200 fordetermining an amount of chert in a geological formation. In someaspects, method 200 may be performed with or by the system 100 shown inFIG. 1. Method 200 begins at step 202, which includes identifyinglogging data for a selected zone of a first subterranean formation thatcomprises chert, quartz, and zircon. For example, as described, thesubterranean formation 110 may be a formation, such as a Qusaiba shaleformation, that is comprised of chert, quartz, and zircon (e.g.,according to known morphology). The identified logging data, which maybe stored or previously stored, or received directly from a logging toolwithin a wellbore, may provide (e.g., by density measurements), avolumetric log of the quartz in the formation (e.g., according to depth)as well as a volumetric log of the zircon in the formation (e.g.,according to depth). For instance, turning briefly to FIG. 3, graph 300illustrates logging data and calculated values used in the examplemethod 200. Column 302 includes log 312 that shows an amount of quartzby volume (according to depth) in the subterranean formation, whichincludes a selected zone of the formation (e.g., zone 112 of formation110). Column 304 includes log 314 that shows an amount of zircon byvolume (according to depth) in the subterranean formation, whichincludes the selected zone of the formation (e.g., zone 112 of formation110).

Method 200 continues at step 204, which includes identifying loggingdata for a second subterranean formation that is different than thefirst subterranean formation. For example, as described, normalizedlogging data may be taken from a subterranean formation that isdifferent (e.g., different known morphology, different location, etc.)from the selected subterranean formation and selected zone within theformation.

Method 200 continues at step 206, which includes determining a firstvolumetric ratio of quartz to zircon in the selected zone based on thelogging data of the first subterranean zone. For example, as shown inFIG. 3, column 306 shows a quartz to zircon volume ratio log 316according to depth in the wellbore. This ratio can be calculated, forexample, according to logs 312 and 314, which show quartz volume andzircon volume, respectively, according to depth in the wellbore fromwhich the logging data originated.

Method 200 continues at step 208, which includes determining a secondvolumetric ratio of quartz to zircon based on the logging data of thesecond subterranean zone. For example, the normalized logging data, muchlike the logging data from the first subterranean zone shown in FIG. 3,can include quartz and zircon volume logs according to depth, which canbe used to develop a quartz to zircon volume ratio for the unrelatedsubterranean formation. FIG. 3 shows the normalized quartz to zirconvolume ratio log 318 in volume 306. In some aspects, the log 318represents a zero or “no chert” line that can be used as a scale ratiofor the quartz to zircon volume ratio log 316. For example, in someaspects, the log 318 may be used to identify a background response ofthe subterranean formation to be drilled (or already drilled). Thesecond volumetric ratio (e.g., log 318) can be determined by selectingan average quartz to zircon ratio reading of subterranean formationsthat are not within the selected formation (e.g., formation 110) andselected zone (e.g., zone 112) of the formation. In some aspects, suchindependent subterranean formations are not within marine depositionalenvironments or include a particular source rock hydrocarbon bearingformation, such as, in this example, Qusaiba shale.

Method 200 continues at step 210, which includes determining a maximumvalue of the second volumetric ratio. For example, the maximum value ofthe second volumetric ratio can be determined by maximizing thenormalized quartz to zircon volume ratio for the unrelated subterraneanformation based on the normalized logging data.

Method 200 continues at step 212, which includes calculating avolumetric percentage of chert in the selected zone based on the firstand second volumetric ratios and the maximum value of the secondvolumetric ratio. In some aspects, the volumetric percentage of chert inthe selected zone (e.g., for the Qusaiba source rock formation) is basedon the equation:

${P_{chert} = \frac{R_{zone} - R_{Baseline}}{R_{\max}}},$where P_(chert) is the volumetric percentage of the chert (shown in FIG.3 as log 324 in column 308) in the selected zone, R_(zone) is the firstvolumetric ratio, R_(Baseline) is the second volumetric ratio, andR_(max) is the maximum value of the second volumetric ratio.

Method 200 continues at step 214, which includes calculating an absolutevolume of the chert in the selected zone based on the volumetricpercentage of the chert in the selected zone. For example, the absolutevolume of the chert in the selected zone can be determined bymultiplying the volumetric percentage of the chert in the selected zonedetermined in step 212 by the volume of quartz in the selected zone(e.g., log 312).

Method 200 continues at step 216, which includes displaying, on agraphical user interface, the calculated volumetric percentage of chertin the selected zone. For example, in some aspects, the graphicaldisplay 300 may be displayed to the user, e.g., in real-time duringlogging or logging-while-drilling, or subsequent to these operations.

Method 200 continues at step 218, which includes a determination ofwhether a calculated volumetric percentage or absolute volume of chertexceeds a threshold value. For example, in some aspects, a driller orother entity associated with drilling and/or completion of a hydrocarbonwell may desire to avoid drilling or completing (e.g., fracturing)through chert when possible. For instance, the presence of chert in thesubterranean formation and selected zone (e.g., forproduction/completion operations) may affect the drilling and, in somecases, a horizontal well placement. When planning horizontal wellsacross a formation that includes chert, penetration through theformation (e.g., up, down, across) may be difficult and require severalbit changes to complete the drilling due to the hardness of chert. Asfor completion operations, such as hydraulically fracturing,unconventional or tight formations that contain a high volumetric ratioof chert may be difficult to break or fracture with an acceptably highfracture efficiency. Further, any fracture growth may be limited once ithits the chert in the formation or zone. Thus, during drilling andcompletion operations, it may be preferable to avoid a subterraneanformation or selected zone that includes chert in a volumetricpercentage above a predetermined threshold.

FIG. 3, for instance, shows effects that the presence of chert may haveon drilling properties, such as rate of penetration (ROP) and weight onbit (WOB). Column 310 shows drilling properties of an example drillingoperation through the first subterranean formation, including a ROP log326 and WOB log 328. As illustrated in column 310, although the ROPstays fairly constant in the presence of chert (shown by log 324), theWOB increases with increasing amounts of chert in the subterraneanformation. Thus, the drilling operation is less efficient (e.g., moreWOB is required to drill at the same ROP) in the presence of increasingvolumetric ratios of chert within the subterranean formation.

Method 200 continues at step 220, which includes recommending anadjustment to a drilling or completion operation based on the calculatedvolumetric percentage of the chert. For example, with knowledge of thevolumetric ratio of chert, recommendations may be made to, e.g., drillin other locations or depths, abandon a drilling operation or fracturingoperation, relocate a fracturing operation so that the chert is notbetween the wellbore and the hydrocarbon bearing selected zone of thesubterranean formation.

FIG. 4 is a graph that illustrates rock strength measurements using ascratch test performed on a core sample of a rock formation thatcontains chert used in a laboratory test to confirm the example methodof FIG. 2. For example, the accuracy of the results of method 200 wereevaluated by using data from labs and field to validate the methodologyof determining an amount of chert according to FIG. 2 and the presentdisclosure. For example, core samples from different wells in a Qusaibasource rock formation were evaluated in labs. The laboratory measurementof rock strength showed high rock strength values across the zones withchert. Uniaxial compressive strength and laboratory scratch tests showabnormally hard rock in chert formations which also included organicmatter, sandstone, carbonates, and different types of clays.

The chart 400 of FIG. 4 shows the results of these laboratory tests onthe core samples from the Qusaiba formation. Chart 400 includes raw logdata 402, which shows, over a depth of a wellbore from which the coresamples were taken. The raw log data 402 shows logging data (e.g.,density, neutron and gamma ray) from the source formation. The lithologylog 404 shows the formation composition; in this case, a clasticformation with source rock (kerogen) content including other minerals ofquartz, chert, illite, chlorite, kaolinite, calcite and albite. Chart400 also includes a deposition thorium-uranium 230 dating (“TH/U”) log406, which is shown here to indicate that the source formation is amarine depositional environment. For example, TH/U data can show an ageof calcium carbonate materials such as speleothem or coral in a marinedepositional environment. Finally, chart 400 includes a rock strengthlog 408 which shows the results of laboratory scratch test rock strengthof the source formation core samples. As shown in chart 400, and therock strength log 408 in particular, although the raw log data 402 doesnot indicate any major changes across the formation which includeschert, the rock strength log 408 indicates (through a greater rockstrength indication) chert in the formation.

FIG. 5 is a photomicrograph 500 of a source rock formation that containschert that illustrates a laboratory test to confirm the example methodof FIG. 2. For example, the photomicrograph 500 was taken of a coresample of the Qusaiba formation to detect a presence of chert in theformation, even though logging data (which did not distinguish betweenquartz and chert) did not show the presence of chert in the samples. Thephotomicrograph 500, as highlighted in the callout box, shows thepresence of a microcrystalline authigenic quartz layer in the coresample (from a formation in which chert was quantified using method200). In some aspects, the presence of the microcrystalline authigenicquartz layer is an indication of chert in the formation.

FIG. 6 illustrates focus ion beam and scanning electron microscopyimages 602 and 604 from samples of a rock formation that contains chertused in a laboratory test to confirm the example method of FIG. 2. Forexample, the images 602 and 604 were taken of core samples of theQusaiba formation to detect a presence of chert in the formation. Theimages 602 and 604 were taken from two core samples from the same wellformed in the same subterranean formation. Image 602 is of a core samplefrom a shallower portion of the wellbore relative to a core sample shownin the image 604. The image 602 shows that the shallower core sample hadcrushed pores (shown as black portions of the image) due to anoverburden and small matrix support. The image 604 shows that the deepersample has chert identified, and the chert, as a known high strengthrock, protected the pores (circular shaped black portions) from beingcrushed. Indeed, image 604 shows almost perfect circular poressurrounded by chert, which gave the rock matrix support against theoverburden and prevented the pores from being crushed in the organicmatter.

FIG. 7 illustrates a schematic diagram of a computing system for acomputer-implemented method such as method 200 shown in FIG. 2. Thesystem 700 can be used for the operations described in association withany of the computer-implemented methods described previously, forexample as the control system 122 that is included within the wellboresystem 100 shown in FIG. 1.

The system 700 is intended to include various forms of digitalcomputers, such as laptops, desktops, workstations, personal digitalassistants, servers, blade servers, mainframes, and other appropriatecomputers. The system 700 can also include mobile devices, such aspersonal digital assistants, cellular telephones, smartphones, and othersimilar computing devices. Additionally, the system can include portablestorage media, such as, Universal Serial Bus (USB) flash drives. Forexample, the USB flash drives may store operating systems and otherapplications. The USB flash drives can include input/output components,such as a wireless transmitter or USB connector that may be insertedinto a USB port of another computing device.

The system 700 includes a processor 710, a memory 720, a storage device730, and an input/output device 740. Each of the components 710, 720,730, and 740 are interconnected using a system bus 750. The processor710 is capable of processing instructions for execution within thesystem 700. The processor may be designed using any of a number ofarchitectures. For example, the processor 710 may be a CISC (ComplexInstruction Set Computers) processor, a RISC (Reduced Instruction SetComputer) processor, or a MISC (Minimal Instruction Set Computer)processor.

In one implementation, the processor 710 is a single-threaded processor.In another implementation, the processor 710 is a multi-threadedprocessor. The processor 710 is capable of processing instructionsstored in the memory 720 or on the storage device 730 to displaygraphical information for a user interface on the input/output device740.

The memory 720 stores information within the system 700. In oneimplementation, the memory 720 is a computer-readable medium. In oneimplementation, the memory 720 is a volatile memory unit. In anotherimplementation, the memory 720 is a non-volatile memory unit. In someimplementations, the control modules herein may not include a memorymodule 720.

The storage device 730 is capable of providing mass storage for thesystem 700. In one implementation, the storage device 730 is acomputer-readable medium. In various different implementations, thestorage device 730 may be a floppy disk device, a hard disk device, anoptical disk device, or a tape device.

The input/output device 740 provides input/output operations for thesystem 700. In one implementation, the input/output device 740 includesa keyboard and/or pointing device. In another implementation, theinput/output device 740 includes a display unit for displaying graphicaluser interfaces.

The features described can be implemented in digital electroniccircuitry, or in computer hardware, firmware, software, or incombinations of them. The apparatus can be implemented in a computerprogram product tangibly embodied in an information carrier, forexample, in a machine-readable storage device for execution by aprogrammable processor, and method steps can be performed by aprogrammable processor executing a program of instructions to performfunctions of the described implementations by operating on input dataand generating output. The described features can be implementedadvantageously in one or more computer programs that are executable on aprogrammable system including at least one programmable processorcoupled to receive data and instructions from, and to transmit data andinstructions to, a data storage system, at least one input device, andat least one output device. A computer program is a set of instructionsthat can be used, directly or indirectly, in a computer to perform acertain activity or bring about a certain result. A computer program canbe written in any form of programming language, including compiled orinterpreted languages, and it can be deployed in any form, including asa stand-alone program or as a module, component, subroutine, or otherunit suitable for use in a computing environment.

Suitable processors for the execution of a program of instructionsinclude, by way of example, both general and special purposemicroprocessors, and the sole processor or one of multiple processors ofany kind of computer. Generally, a processor will receive instructionsand data from a read-only memory or a random access memory or both. Theessential elements of a computer are a processor for executinginstructions and one or more memories for storing instructions and data.Generally, a computer will also include, or be operatively coupled tocommunicate with, one or more mass storage devices for storing datafiles. Such devices include magnetic disks, such as internal hard disksand removable disks, magneto-optical disks, and optical disks. Storagedevices suitable for tangibly embodying computer program instructionsand data include all forms of non-volatile memory, including by way ofexample semiconductor memory devices, such as EPROM, EEPROM, and flashmemory devices, magnetic disks such as internal hard disks and removabledisks, magneto-optical disks, and CD-ROM and DVD-ROM disks. Theprocessor and the memory can be supplemented by, or incorporated in,ASICs (application-specific integrated circuits).

To provide for interaction with a user, the features can be implementedon a computer having a display device such as a CRT (cathode ray tube)or LCD (liquid crystal display) monitor for displaying information tothe user and a keyboard and a pointing device such as a mouse or atrackball by which the user can provide input to the computer.Additionally, such activities can be implemented via touchscreenflat-panel displays and other appropriate mechanisms.

The features can be implemented in a control system that includes aback-end component, such as a data server, or that includes a middlewarecomponent, such as an application server or an Internet server, or thatincludes a front-end component, such as a client computer having agraphical user interface or an Internet browser, or any combination ofthem. The components of the system can be connected by any form ormedium of digital data communication such as a communication network.Examples of communication networks include a local area network (“LAN”),a wide area network (“WAN”), peer-to-peer networks (having ad-hoc orstatic members), grid computing infrastructures, and the Internet.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular implementations of particularinventions. Certain features that are described in this specification inthe context of separate implementations can also be implemented incombination in a single implementation. Conversely, various featuresthat are described in the context of a single implementation can also beimplemented in multiple implementations separately or in any suitablesubcombination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. For example, exampleoperations, methods, and/or processes described herein may include moresteps or fewer steps than those described. Further, the steps in suchexample operations, methods, and/or processes may be performed indifferent successions than that described or illustrated in the figures.Accordingly, other implementations are within the scope of the followingclaims.

What is claimed is:
 1. A computer-implemented method for determining anamount of chert in a rock formation, comprising: identifying firstlogging data for a selected zone of a first subterranean formation thatcomprises chert, quartz, and zircon; identifying second logging data fora second subterranean formation that is different than the firstsubterranean formation, the second subterranean formation comprisingchert, quartz, and zircon; determining a first volumetric ratio ofquartz to zircon in the selected zone based on the first logging data;determining a second volumetric ratio of quartz to zircon based on thesecond logging data; determining a maximum value of the secondvolumetric ratio; calculating, based on the first and second volumetricratios and the maximum value of the second volumetric ratio, avolumetric percentage of chert in the selected zone; and displaying, ona graphical user interface, the calculated volumetric percentage ofchert in the selected zone, where displaying the calculated volumetricpercentage of chert in the selected zone comprises displaying thecalculated volumetric percentage of chert as a function of depth betweena shallowest depth of the selected zone and a deepest depth of theselected zone.
 2. The computer-implemented method of claim 1, furthercomprising: calculating an absolute volume of the chert in the selectedzone based on the volumetric percentage of the chert in the selectedzone.
 3. The computer-implemented method of claim 2, wherein calculatingthe absolute volume of the chert in the selected zone comprises:determining a total volume of the quartz in the selected zone; andmultiplying the volumetric percentage of the chert in the selected zoneby the total volume of the quartz in the selected zone.
 4. Thecomputer-implemented method of claim 1, wherein the first subterraneanformation comprises a marine deposition subterranean environment, andthe second subterranean formation comprises a non-marine depositionsubterranean environment.
 5. The computer-implemented method of claim 1,wherein calculating, based on the first and second volumetric ratios andthe maximum value of the second volumetric ratio, a volumetricpercentage of the chert in the selected zone comprises solving theequation: ${P_{chert} = \frac{R_{zone} - R_{Baseline}}{R_{\max}}},$where P_(chert) is the volumetric percentage of the chert in theselected zone, R_(zone) is the first volumetric ratio, R_(Baseline) isthe second volumetric ratio, and R_(max) is the maximum value of thesecond volumetric ratio.
 6. The computer-implemented method of claim 1,wherein the selected zone comprises a Qusaiba geological formation. 7.The computer-implemented method of claim 1, further comprisingrecommending an adjustment to a drilling or completion operation basedat least in part on the calculated volumetric percentage of chert in theselected zone.
 8. The computer-implemented method of claim 1, furthercomprising receiving the first logging data from a logging tool in awellbore formed through the selected zone of the first subterraneanformation.
 9. The computer-implemented method of claim 8, wherein thelogging tool comprises a logging-while-drilling (LWD) tool.
 10. Thecomputer-implemented method of claim 9, further comprising: calculatingan absolute volume of the chert in the selected zone based on thevolumetric percentage of the chert in the selected zone.
 11. Thecomputer-implemented method of claim 10, wherein calculating theabsolute volume of the chert in the selected zone comprises: determininga total volume of the quartz in the selected zone; and multiplying thevolumetric percentage of the chert in the selected zone by the totalvolume of the quartz in the selected zone.
 12. A system, comprising: oneor more hardware processors; and one or more memory modules that storeinstructions executable by the one or more hardware processors toperform operations comprising: identifying first logging data for aselected zone of a first subterranean formation that comprises chert,quartz, and zircon; identifying second logging data for a secondsubterranean formation that is different than the first subterraneanformation, the second subterranean formation comprising chert, quartz,and zircon; determining a first volumetric ratio of quartz to zircon inthe selected zone based on the first logging data; determining a secondvolumetric ratio of quartz to zircon based on the second logging data;determining a maximum value of the second volumetric ratio; calculating,based on the first and second volumetric ratios and the maximum value ofthe second volumetric ratio, a volumetric percentage of chert in theselected zone; and displaying, on a graphical user interface, thecalculated volumetric percentage of chert in the selected zone, wheredisplaying the calculated volumetric percentage of chert in the selectedzone comprises displaying the calculated volumetric percentage of chertas a function of depth between a shallowest depth of the selected zoneand a deepest depth of the selected zone.
 13. The system of claim 12,wherein the operations further comprise: calculating an absolute volumeof the chert in the selected zone based on the volumetric percentage ofthe chert in the selected zone.
 14. The system of claim 13, whereincalculating the absolute volume of the chert in the selected zonecomprises: determining a total volume of the quartz in the selectedzone; and multiplying the volumetric percentage of the chert in theselected zone by the total volume of the quartz in the selected zone.15. The system of claim 12, wherein the first subterranean formationcomprises a marine deposition subterranean environment, and the secondsubterranean formation comprises a non-marine deposition subterraneanenvironment.
 16. The system of claim 12, wherein calculating, based onthe first and second volumetric ratios and the maximum value of thesecond volumetric ratio, a volumetric percentage of the chert in theselected zone comprises solving the equation:${P_{chert} = \frac{R_{zone} - R_{Baseline}}{R_{\max}}},$ whereP_(chert) is the volumetric percentage of the chert in the selectedzone, R_(zone) is the first volumetric ratio, R_(Baseline) is the secondvolumetric ratio, and R_(max) is the maximum value of the secondvolumetric ratio.
 17. The system of claim 12, wherein the selected zonecomprises a Qusaiba geological formation.
 18. The system of claim 12,wherein the operations further comprise recommending an adjustment to adrilling or completion operation based at least in part on thecalculated volumetric percentage of chert in the selected zone.
 19. Thesystem of claim 12, further comprising a logging tool communicablycoupled with the one or more hardware processors, and the operationsfurther comprise receiving the first logging data from the logging toolin a wellbore formed through the selected zone of the first subterraneanformation.
 20. The system of claim 19, wherein the operations furthercomprise receiving the first logging data from the logging tool duringthe formation of the wellbore through the selected zone of the firstsubterranean formation.
 21. The system of claim 20, wherein theoperations further comprise: calculating an absolute volume of the chertin the selected zone based on the volumetric percentage of the chert inthe selected zone.
 22. The system of claim 21, wherein calculating theabsolute volume of the chert in the selected zone comprises: determininga total volume of the quartz in the selected zone; and multiplying thevolumetric percentage of the chert in the selected zone by the totalvolume of the quartz in the selected zone.